For many years, cathode ray tube (CRT) displays have been incorporated into a wide variety of systems such as television receivers, computers, and civilian and military control and monitoring systems, including avionic and aerospace control and monitoring systems. CRT displays have several disadvantages. CRT displays require a large amount of space, consume a great deal of electrical power and are heavy. These disadvantages are incompatible with the modern desire to make the foregoing and other electronic display systems smaller, lighter, and more durable.
In order to meet the need for smaller, lighter and more durable electronic displays, liquid crystal displays (LCDs) and, more recently, active matrix liquid crystal displays (AMLCDs) have been developed. LCDs and AMLCDs are flat panel displays that include a sandwich made of a liquid crystal layer located between front and rear polarizers. Located behind the sandwich is a backlight that includes a reflector, a lamp, and a diffuser. The liquid crystal material layer includes liquid crystal cells that either allow polarized light to pass through unaffected or rotate the orientation of the polarized light by up to ninety degrees (90.degree.). An electric field controls the alignment of the liquid crystal modules and, thus, LCD images. AMLCDs overcome some of the scan limitations of LCDs. More specifically, LCD mages are controlled by addressing a matrix of x-y electrodes that define a pixel array. The voltage applied to the x-y electrodes controls the display image. An image is created as the rows of pixels are sequentially scanned. The most common LCD material--twisted-nematic field-effect (TNFE)--is not bi-stable and begins to relax immediately after it has been addressed, resulting in a low contrast display. AMLCDs do not have this problem because AMLCDs contain active electronic elements within each pixel that switch and hold the voltage on a pixel until the entire matrix is scanned.
LCDs and AMLCDs are used extensively in products varying from laptop computers to military helicopters. LCDs and AMLCDs have a number of qualities that make them better than CRT displays in many environments. First, they have a low profile, taking up a fraction of the space of conventional CRT displays. Second, AMLCDs and LCDs weigh considerably less than CRT displays. These qualities make AMLCDs and LCDs highly desirable for use in commercial and military aircraft and in other environments. Unfortunately, LCDs and AMLCDs have relatively low light transmission--3% to 10% in the case of AMLCDs. Under high ambient light conditions (e.g., sunlight), more than 150 fL of net LCD or AMLCD luminance is needed for a display to be viewable. This amount of display luminance requires a backlight capable of producing at least 5,000 fL.
Recent improvements in AMLCD and LCD technology have focused on a number of problem areas. One area of concern is noted above--viewability in high ambient sunlight conditions. Another important area of concern is uniform light distribution over the entire display area. Not only must the light be bright, it also must be uniform. Uniform light distribution means that the lamp geometry of the backlight assembly of an AMLCD or an LCD is not apparent to a user. Nonuniform light distribution causes the appearance of "bright" and "dim" areas in the display.
Another area of concern in some AMLCD and LCD environments is providing adequate off-axis luminance so that a viewer at a known, specific location that is significantly off the perpendicular or head-on viewing angle of the screen can view the image being displayed. For example, in a Cobra helicopter used by the U.S. military, one display is located approximately at chest height relative to the pilot. Rather than being tilted, the display is oriented such that the perpendicular line of sight of the display hits the pilot in the upper chest. While CRT displays with a near Lambertian light distribution characteristic positioned and oriented in this manner will be easily viewed by a Cobra pilot, previously developed AMLCDs and LCDs positioned and oriented in the same manner have been difficult to view due to their limited viewing angle.
A final area of concern in some AMLCD and LCD environments is the compatibility of backlight optics with the night vision goggles (NVGs) used during night military aircraft operations. In order to meet the stringent infrared radiance requirements given by military specification MIL-L-85762A, backlit AMLCDs and LCDs require the incorporation of an infrared filter into the backlight assembly.
U.S. patent application Ser. No. 08/576,767 filed Dec. 21, 1995, titled ILLUMINATION OPTICS FOR BACKLIT DISPLAYS by Leonard Y. Nelson et al., the subject matter of which is incorporated herein by reference, is directed to addressing the foregoing concerns by providing high quality, uniform wide angle illumination optics for backlit displays, such as signs, LCDs and AMLCDs. The present invention is directed to improving illumination optics for backlit displays of the type described in U.S. patent application Ser. No. 08/576,767, designed for use with AMLCDs, LCDs or other displays intended to be compatible with NVGs.